Since the pioneering inventions of Strong et al., disclosed in British Patent No. 690,691, published Apr. 29, 1953, printed circuit board (PCB) technology has become an extremely important facet of the modern electronics industry, PCB technology has been facilitated by the development of cladding and related electrodeposition processes for producing thin foils for use as conductor materials. In an effort to minimize the space requirements on PCB components, the industry has turned to planar resistor technology, and in particular to electrodeposited resistive layers in an effort to increase circuit density, improve reliability and operating characteristics and reduce overall cost, especially by increasing manufacturing automation. These prior activities are summarized in an article of Mahler entitled "Planar Resistor Technology for High-Speed Multilayer Boards", Electronic Packaging & Production, January, 1986, pp 151-154.
Important advances in the field of etchable electrodeposited layers of resistive materials have been made by Castonguay, as disclosed in the U.S. Pat. No. 3,857,683, by Castonguay et al., as disclosed in U.S. Pat. No. 3,808,576 and by Rice et al., as disclosed in International Application No. PCT/US86/01173, published as International Publication No. WO 86/07100.
A large number of binary alloys are disclosed in U.S. Pat. No. 3,857,683 as being useful as resistive layers. However, most of these alloys have specific resistivities which are too low for practical commercial uses. Moreover, in many cases the ingredients necessary in the respective electroplating baths for producing the alloys are exotic and expensive and in some cases difficult and/or dangerous to handle. For example, in Examples XI and XXXI of the '683 patent, fluoborates of antimony and either cobalt or nickel and fluoboric acid are needed to prepare cobalt.antimony and nickel.antimony alloys respectively. Additionally, this prior publication provides no guidance whatsoever with regard to particular etchants or etching conditions which might be successfully employed to prepare printed circuits from a given binary alloy.
In any event, for one reason or another, and as illustrated in U.S. Pat. No. 3,808,576 and International Publication No. WO 86/07100, the nickel.phosphorous alloys have become the most widely used materials for PCB precursor resistive layer applications. In the 07100 International Publication, nickel.phosphorous resistance layers are plated onto copper foil from a plating bath free of sulfate and chloride salts. This reference specifically teaches that the use of such salts should be avoided because they cause embrittlement. Thus, the plating bath comprises nickel carbonate, phosphoric acid and phosphorous acid. In the '576 patent, the resistive layer includes up to 30% by weight phosphorous and the bath from which it is electrodeposited contains nickel sulfate hexahydrate, nickel chloride hexahydrate, nickel carbonate, phosphoric acid and phosphorous acid, as well as other additives. The use of so many reagents for forming the nickel.phosphorous layer, however, is technically cumbersome and tends to increase the cost of producing a commercially suitable resistive layer. Moreover, it is generally thought to be necessary to anodize the resistive alloy into oxides after the same has been electroplated onto a copper foil surface. This also results in added cost and inconvenience.
Thus, in spite of the advances in the art of providing electrodeposited planar resistance layers, those skilled in such art have continued to search for resistive materials which might be electrodeposited conveniently and reproducibly to provide a resistive layer which is readily etchable using safe etchants to provide printed circuit components having resistance lines and segments that have commercially appropriate and valuable resistance characteristics. The invention disclosed in the present application provides such resistive layers, components which incorporate such layers and methods and plating baths for making the same.